git submodule deinit <name_of_submodule>
git rm -f <name_of_submodule>
rm -rf .git/modules/<name_of_submodule>
git commit -m "Deleted submodule xyz"
git submodule deinit <name_of_submodule>
git rm -f <name_of_submodule>
rm -rf .git/modules/<name_of_submodule>
git commit -m "Deleted submodule xyz"